This invention relates to a radar apparatus equipped with a plurality of antennas and having a function for electronically controlling antenna beams. More particularly, the invention relates to a radar apparatus in which it is sufficient to provide one front end commonly for the antennas.
There are three conventional methods available for controlling an antenna beam. The first method involves changing the direction of the antenna mechanically. The second method involves switching among a plurality of antennas having different directivities. The third method entails arraying a number of antennas and combining the antenna radiation patterns electrically.
A number of problems arise with mechanical beam control. Specifically, the apparatus is large in size owing to the need for a drive unit. In a case where the antennas are integrated and the transceiver is heavy, the required accuracy cannot be obtained unless the drive unit used is complex and large in size. In addition, high-speed sweep is not possible. For these reasons, the selection made most often is electronic beam control means.
Methods of electronic beam control that can be mentioned are the aforementioned method of switching among a plurality of antennas having different directivities and the method of arraying a number of antennas and combining the antenna radiation patterns electrically. The former is referred to as beam switching. The latter is typified by an active phased array antenna in which transmission signals to individual antennas or reception signals from individual antennas are combined after being adjusted in terms of phase and amplitude. Consider a radar apparatus equipped with a receiver unit in which antenna beams are capable of being controlled.
A receiver unit of the beam-switching type (1) includes a switch connected directly to a plurality of antennas and adapted to switch among the antennas so as to connect them to a radio-frequency receiver (referred to as an "RF receiver circuit" below), which is the next stage, or (2) includes a plurality of antennas, RF receiver circuits connected directly to respective ones of these antennas, and a switch for switching among these RF receiver circuits so as to connect them to a processing unit, which is the next stage. With beam switching, the individual antennas are provided with directivities that differ from one another and the direction to be investigated by radar is controlled by switching among these antenna using an electronic switch. The RF receiver circuit subjects the received signal to low-noise amplification and frequency conversion. When the noise characteristic of the receiver is taken into consideration, the arrangement (2) mentioned above, namely that in which the antennas and RF receiver circuits are directly connected, is adopted.
As shown in FIG. 24, a receiver unit that employs the active phased array technique comprises a plurality of arrayed antennas 1a.about.1n, an RF receiver 2 having RF receiver circuits 2a.about.2n connected directly to the antennas 1a.about.1n, respectively, and an adder 10 for adding the signals from the RF receiver circuits 2a.about.2n in order to combine them. The RF receiver circuits 2a.about.2n function to amplify and frequency-convert the received signals from the respective antennas and to perform a phase shift and amplitude adjustment suited to a desired beam pattern.
The RF receiver 2 includes, in addition to the RF receiver circuits 2a.about.2n connected directly to the antennas, a phase control circuit 7 for deciding the amount of phase shift, an amplitude control circuit 8 for deciding the amount of amplitude adjustment, and a local oscillator 9. The RF receiver circuits 2a.about.2n respectively include RF amplifiers 3a.about.3n for low-noise amplification of RF signals received from the corresponding antennas, phase shifters 4a.about.4n for applying phase shifts of prescribed amounts .phi..sub.1 .about..phi..sub.N to the corresponding low-noise amplified RF signals, amplitude adjusters 5a.about.5n for adjusting the amplitudes of the signals output by the corresponding phase shifters, and mixers (frequency converters) 6a.about.6n for converting the RF signals, which are output by the corresponding amplitude adjusters, to intermediate-frequency (IF) signals. The phase control circuit 7 and amplitude control circuit 8 decide the phase shift quantities .phi..sub.1 .about..phi..sub.N and amplitude adjustment values A.sub.1 .about.A.sub.N so as to perform a phase shift and amplitude adjustment suited to the desired beam pattern, and input the phase shift quantities .phi..sub.1 .about..phi..sub.N to the respective phase shifters 4a.about.4n and the amplitude adjustment values A.sub.1 .about.A.sub.N to the respective amplitude adjusters 5a.about.5n. The local oscillator 9 oscillates at a predetermined frequency and inputs the local oscillation signal to each of the mixers 6a.about.6n. The adder 10 combines the outputs of the mixers and inputs the result to a processor, which is not shown.
With the receiver unit that employs the active phased array technique, the combined radiation pattern is controlled by controlling the amount of phase shift and the amount of amplitude adjustment of each antenna reception signal, thus making it possible to change the direction of radar detection. Further, by changing the amount of phase shift and the amount of amplitude adjustment continuously, continuous control of the direction of radar detection also becomes possible.
The foregoing is an example of an analog configuration. However, by making use of digital technology, it is possible to realize a DBFN (Digital Beam Forming Network) having both digital and analog characteristics. A DBFN has a configuration similar to that of the active phased array arrangement, as illustrated in FIG. 25. The apparatus of FIG. 25 differs from that of FIG. 24 in that (1) the frequency converters 6a.about.6n and local oscillator 9 are disposed at the outputs of the RF amplifiers 3a.about.3n, (2) IF filters 101a.about.101n are provided for extracting intermediate-frequency components from the output signals of the frequency converters 6a.about.6n, respectively, (3) A/D converters 102a.about.102n are provided for converting the analog outputs of the IF filters to digital data, and (4) control of phase shift and amplitude is executed by digital processing using a DSP (Digital Signal Processor) or the like.
The DBFN-type receiver apparatus is so adapted that reception signals from the antennas 1a.about.1n are amplified by the RF amplifiers 3a.about.3n, respectively, and then subjected to a frequency conversion by the frequency converters 6a.about.6n, respectively. The IF filters 101a.about.101n extract intermediate-frequency components from the outputs of the frequency converters 6a.about.6n, respectively, and the A/D converters 102a.about.102n convert the IF signals to digital data. The DSP controls the amount of phase shift and adjusts amplitude by digital processing, and the adder 10 combines the resulting signals in terms of their vectors. As a result of this operation, a desired beam pattern is formed and the direction of detection is controlled. This is a principle exactly the same as that of the active phased array configuration described above.
Further, as shown in FIG. 26, an arrangement may be adopted in which a plurality of phase-shift/amplitude-adjustment means 103a.about.103m are provided, digital data output by the A/D converters 102a.about.102n is input to each of the phase-shift/amplitude-adjustment means 103a.about.103m, these input signals are subjected to separate phase shifts and amplitude adjustments by each of the phase-shift/amplitude-adjustment means 103a.about.103m and each of the phase-shift/amplitude-adjustment means 103a.about.103m combines the resulting signals and outputs the result. Adopting this arrangement makes it possible to form a plurality of emission patterns simultaneously so that signals from a plurality of directions can be distinguished among and obtained simultaneously. In the arrangement of FIG. 26, the active phased array technique and the beam switching technique can be realized at the same time. It should be noted that the reason for using digital circuitry is ease of design and manufacture.
A scheme in addition to the active phased array system that can be used to detect the direction of a target is the monopulse technique. Unlike the active phased array configuration, the monopulse scheme receives reflected power from a target by two antennas and compares the phases or amplitudes of the reflected power received by the two antennas to thereby estimate and detect the direction of the target (the signal arrival direction).
FIG. 27 is a diagram showing the construction of the receiver in a monopulse radar apparatus. The receiver includes antennas 110a, 110b, low-noise RF amplifiers 111a, 111b, local oscillators 112a, 112b, frequency converters 113a, 113b, IF filters 114a, 114b, A/D converters 115a, 115b for converting IF signals to digital data, a phase comparator circuit 116 for comparing the phases of signals received by the two antennas, an amplitude comparator circuit 117 for comparing the amplitudes of the signals received by the two antennas, and an arrival direction estimating circuit 118 for estimating the direction of a target (the signal arrival direction) based upon a phase difference or amplitude distance.
Though the two receiving antennas 110a, 110b point in substantially the same direction, the positions at which they are placed differ slightly. Consequently, the radiation beam patterns overlap with a slight offset between them. If the target is at equal distances from both antennas, the phases of the received signals that arrive at the two antennas 110a, 110b will be equal. If the target is closer to one antenna than the other, however, the signal arrival direction (the target direction) can be estimated from the phase difference between the two signals arriving at the respective antennas and the spacing between the antennas. The monopulse radar apparatus estimates the direction of the target in accordance with this principle.
The beam switching arrangement in which the antennas are switched among necessitates a plurality of independent antennas. It is required that the individual beam widths be comparatively small. In addition, depending upon the radar application, it is often required that the beam widths be uniformalized as well as the antenna gains. As a consequence, a plurality of antennas having comparatively large areas are required and, hence, the area occupied by the antennas is several times larger than that of a radar apparatus devoid of beam control or of a radar apparatus of the active phased array type. The cost of manufacturing the antenna components is high as well. Further, according to the beam switching system, control of detection direction is performed in discrete fashion by switching among the plurality of antennas having different directivities. As a result, the angular resolution of the target is limited by the beam widths of the individual antennas and the number of antennas.
With the active phased array and DBFN systems, a plurality of antenna radiation patterns are combined to obtain a single radiation pattern having the desired directivity. This means that it is unnecessary to enlarge the size of the antennas, as is required in the beam switching system. Further, by continuously varying the amount of phase shift and the amount of amplitude adjustment, continuous control of detection direction becomes possible, as set forth above. This makes it possible to increase angular resolution. However, the active phased array and DBFN systems require the individual RF receiver circuits for the plurality of antennas. This increases the size, complexity and manufacturing cost of the apparatus. Further, a manufacturing variance in the amplitude and phase characteristics specific to the circuitry and a manufacturing variance in the temperature and frequency characteristics of the circuit parameters become more pronounced at higher frequencies. This makes it necessary to take special care in apparatus design, and there are cases where compensating means and adjustment circuits must be provided. In the case of the multiple-beam DBFN arrangement (FIG. 26) having a plurality of beam combining means, a problem which arises is a more complicated apparatus.
Individual RF receiver circuits are necessary for the antennas in the monopulse arrangement as well, leading to a problem similar to that seen with the active phased array configuration.